The publications and other materials used herein to illuminate the background of the invention, and in particular, to provide additional details with respect to the practice, are incorporated by reference.
The α2-adrenoceptors (α2-ARs) mediate many of the physiological effects of the catecholamines norepinephrine and epinephrine. Three genetic subtypes of α2-adrenoceptors are known in humans and other mammals, denoted as α2A-, α2B- and α2C-adrenoceptors. The human genes encoding the receptors are located on chromosomes 10, 2 and 4, respectively. No splice variants are known to exist of these receptors, as the genes are intronless. The tissue distributions and physiological and pharmacological functions of the receptor subtypes have been reviewed e.g. by MacDonald et al. (1997) and Docherty (1998). Based on recent studies with gene-targeted and transgenic mice, α2A-adrenoceptors mediate most of the pharmacological actions ascribed to currently available α2-adrenoceptor agonists, including inhibition of neurotransmitter release, central hypotensive and bradycardic effects, sedation and anesthesia, and analgesia. The same studies indicate that α2B-adrenoceptors mediate peripheral pressor responses in response to agonist activation (Link et al. 1996, Macmillan et al. 1996) and thus play a significant role in the onset of hypertension (Calzada and Artinano 2001). Other physiological or pharmacological effects have not been associated with certainty with this receptor subtype. The α2C-adrenoceptor subtype appears to be involved in regulation of complex behaviors. It is not known that this subtype would have important functions in peripheral tissues outside the central nervous system or in cardiovascular regulation.
Hypertension, like many other common disorders, arises from complex interactions between genetic and environmental factors. It is reasonable to assume that functionally important genetic variation in mechanisms important for the regulation of vascular functions will be found to be associated with the pathogenesis and therapy of hypertension. A variant form of the human α2B-AR gene was recently identified (Heinonen et al., 1999). The variant allele encodes a receptor protein with a deletion of three glutamate residues in an acidic stretch of 18 amino acids (of which 15 are glutamates) located in the third intracellular loop of the receptor polypeptide. This acidic stretch is a unique feature in the primary structure of α2B-AR in comparison to α2A-AR and α2c-AR, suggesting that the motif has a distinct role in the function of α2B-AR. Amino acid sequence alignment of α2B-AR polypeptides of different mammals reveals that the acidic stretch is highly conserved among the α2B-A of mammals and that the acidic stretch is long in humans in comparison to other species. This suggests that the motif is important for the functionality of the receptor, and that the short form (D for “deletion”) probably represents the ancestral form and the long form (I for “insertion”) could well represent a more recent allelic variant in humans. Jewell-Motz and Liggett (1995) studied the in vitro functions of this stretch using site-directed mutagenesis to delete as well as to substitute 16 amino acids of the stretch. Their results suggest that this acidic motif is necessary for full agonist-promoted receptor phosphorylation and desensitisation.
Based on the vasoconstrictive property of α2B-AR in mice and the involvement of this acidic region in the desensitisation mechanism of the receptor, we hypothesised that the deletion variant confers reduced receptor desensitisation and therefore augmented vasoconstriction of systemic arteries that could be associated with hypertension. To test this hypothesis, we carried out a population study in 912 middle-aged Finnish men.